And one more thing, even though we are supposed too, how many actually do?

It's called checking the output voltage of your BEC.

Come on raise your hands, how many of you actually checked before you plugged in the rest of your system? I thought so.

Yeah, I got burnt on that one. Oh those silly Chinese kids on the assembly line.
I received a BEC that was wired backwards, battery connections were soldered to the output side of the board, and visa versa. Cost me 3 receivers before I finally thought to check. Yep 16.8 volts instead of 6 sure made a mess.

Funny, after catching the boo boo, I used the BEC, in a ground vehicle, and it's been running for a couple of years now.

Yup
I've checked the output voltage of my CC 10 Amp uBEC's on my Tektronixs 2236 oscillocope, especially to sudden application and removal of 10 Amp loads. On three that were checked, they held up output voltage on a momentary 14 Amp load with a 6S A123 battery input. Not bad.

As for checking new stuff, three or four years ago, I had two Castle Creations ESC's where the wires for the servo connector were not properly crimped. They could easily be pulled out of the pins. Found it before the first flights.

Dimension Engineering makes and sells a variety of add-on switch mode BEC's and other electronic devices of interest to modelers and experimenters. Read their FAQ's and installation guides. I have been using their switch-mode Park BEC's in all of my small to medium sized built-up and ARF models with continued success after many unexplained crashes before the dirty little secrets about linear BEC's and 2.4 GHz receiver's voltage requirements became known. Some recent receivers are not as sensitive to dips in voltage and if they do reset/brownout, they may recover quicker letting you regain control. Older receivers, such as DX-6 AR6000's were vulnerable with 3S lipos and four micro servos. Your model crashed but RC system functioned normally after crash unless damaged. BTW, Park BEC's have a "bypass" feature that makes it unnecessary to disconnect the red wire to the receiver.

Dimension Engineering makes and sells a variety of add-on switch mode BEC's and other electronic devices of interest to modelers and experimenters. Read their FAQ's and installation guides. I have been using their switch-mode Park BEC's in all of my small to medium sized built-up and ARF models with continued success after many unexplained crashes before the dirty little secrets about linear BEC's and 2.4 GHz receiver's voltage requirements became known. Some recent receivers are not as sensitive to dips in voltage and if they do reset/brownout, they may recover quicker letting you regain control. Older receivers, such as DX-6 AR6000's were vulnerable with 3S lipos and four micro servos. Your model crashed but RC system functioned normally after crash unless damaged. BTW, Park BEC's have a "bypass" feature that makes it unnecessary to disconnect the red wire to the receiver.

Yeah, I've been very wary about the performance of those "Linear" voltage regulators used on many of the ESC BEC's. IMHO, using them on any motor running on three LiPos or more is just asking for the loss of a model.

OK, I'm going to put an SBEC in the line for the receiver, and what rating would be good for a 2 to 3 cell battery, or for the big plane that takes a 4 cell? From what I've read here, I DON'T have to take the linear BEC in the ESC off the circuit? The SBEC is a failsafe for the receiver? I'm just making sure, I don't want to mess anything up...

No, for most stand alone BEC's you do need to disconnect the built in BEC in the ESC (if fitted). You do this by unhooking the red wire from the Rx plug that comes from the ESC.

The rating of the stand alone BEC must be good for the voltage of your flight battery, but most are good for at least 6 cells so that shouldn't be a problem. It's the output amp rating that's the biggest variable. Personally I think the Castle 10A peak BEC is hard to beat on cost, output and quality. For me it's pointless getting a stand alone BEC with an output less than 5 amps because you can get 4 and 5 amp switch mode BEC's in ESC's these days.

OK, I'm going to put an SBEC in the line for the receiver, and what rating would be good for a 2 to 3 cell battery, or for the big plane that takes a 4 cell? From what I've read here, I DON'T have to take the linear BEC in the ESC off the circuit? The SBEC is a failsafe for the receiver? I'm just making sure, I don't want to mess anything up...

For the most part, it is not a good idea to directly parallel connect two switching uBEC's, they can interact with each other.

If you have a larger model and would like dual battery inputs, they should be isolated from each other with a Shottky diode with a rating of 9 Amps or more. The attached schematic shows how its done. (I've used this in several different models, it works very well.)

As for me, I'd never use a Linear type BEC regulator in any model with more than two LiPo cells for motor power. I've seen to many crashes when the linear BEC overheated, shut down, and, crash. By the time you get to whats left of the model, that linear regulator has cooled off, and is working again.